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Condensation: Dew, Fog, and Clouds

Formation of Dew and Frost• Dew - forms when the temperature cools to the dew-point

temperature – Dew/frost often form only close to the ground, e.g., not on a bush,

why?– Dew and frost most often form on clear, calm nights..., why?

• Dew can be an important source of moisture during periods of low rain fall.

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If T = Td < 32°F, get frost; Dew point is called the frost point.

These are the delicate ice-crystal patterns that frost exhibits on a window during a cold winter morning

Freeze and Black frost: In very dry weather, the air temperature may become quite cold and drop below freezing without ever reaching the frost point, and no visible frost forms.

Condensation NucleiKey for the Formation of Haze, Fog, and Clouds

• The process of condensation of vapor -> water to form a cloud drop is not that simple in the atmosphere

• NEED Condensation Nuclei to form cloud drops• Condensation Nuclei:

– small particles in air created from/by:• dust • volcanoes • factory smoke • forest fires • ocean salt • sulfate particles from phytoplankton in ocean

– They are most abundant in lower troposphere over urban areas

– They are quite small relative to a rain drop or cloud drop

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Sizes and Amounts of CCN• Note: 1 cm3 is about the size of your thumb!• Total mass of CCN put into atmosphere each

year is about 2x1012 kg!

• Two types of CCN:– Hydroscopic - water seeking - H2O readily condenses

on these• ocean salt is a good example (sticky salt shaker when humid)

– Hydrophobic- water repellant - H2O does not readily condense on these (wax on car)

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Formation of Haze• Two types:

– dry haze - large/giant particles in the air (smoke, smog, dust)

– wet haze - H2O condenses onto hydroscopic CCN - can occur at RH's as low as 75%

– wet haze has a dull gray, white color

Fog introduction• Forms as the RH increases to 100% - haze

particles grow into fog (cloud) particles near the ground.

• Fog is really a cloud near the ground

• International definition: Visibility is less than 1 km

• NWS definition: Visibility is less than or equal to 6 miles and T-Td < 5°F

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Fog in polluted areas can be a health problem since it becomes acidic

Formation of Fog• Fog forms in one of two general ways:

– Air is cooled below its saturation point– By evaporation and mixing

• A fog droplet with a diameter of 25 μm settles toward the ground at about 5 cm/s (2 in./s).– Fog is maintained by new fog droplets.

• Types of Fog:– Radiation Fog– Advection Fog– Upslope Fog– Steam Fog

• Let's look at how each of these are formed......

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Formation of Radiation Fog - Radiational Cooling• Need:

– shallow, moist air near surface – clear/calm nights – although light winds will bring more air in contact with

ground • Radiational cooling allows the temperature to drop

to the dew point.

Also called ground fog

Formation of Radiation Fog - T = Td

• Once the temperature reaches the dew point, radiation fog begins to develop– Common in the fall - especially when our weather is

dominated by high pressure– often forms in valleys first since this is where the coldest air

is - called valley fog• Q: Do radiation fogs develop upward or downward? • Q: When is radiation fog the thickest?

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Visible satellite image of dense radiation fog in the southern half of California’s Central Valley on the morning of November 20, 2002. The white region to the east (right) of the fog is the snow-capped Sierra Nevada range. During the late fall and winter, the fog, nestled between two mountain ranges, can last for many days without dissipating. The fog on this day was responsible for several auto accidents, including a 14-car pileup.

Formation of Advection Fog• Common off the west coast of the U.S.

– Cold current along coastline and warm water further to the west

– westerlies moving in from the warmer water to the colder water

– westerlies advect warm moist air over colder water• Heat is transferred from the warm, moist air to the

cold water near the coast via conduction• The parcel reaches saturation - forms fog, and is

advected onshore • NEED a light breeze for this process to occur

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Here advection fog, having formed over the cold, coastal water of the Pacific Ocean, is rolling inland past the Golden Gate Bridge in San Francisco. As fog moves inland, the air warms and the fog lifts above the surface. Eventually, the air becomes warm enough to totally evaporate the fog.

• Advection fog can be an important source of moisture for plant life along the CA coast - it rarely rains there during the summer months

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Other favorite places to observe advection fog

• Where two ocean currents with different temperatures flow next to one another.

• Warm moist air from the gulf of Mexico moves northward over colder land.

• Ice fog in arctic:

Along an irregular coastline, advection fog is more likely to form at the headland where moist surface air converges and rises

than at the beach where air diverges and sinks.

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Upslope Fog• Take a parcel over the Gulf of Mexico and

move it towards Denver...• As the parcel ascends up the slope, it

expands and cools to the dew point• Upslope fog/clouds then form

– Need winds to move air up the slope– Need a slope!

Steam Fog• Seen over lakes or heated pools in winter• Form it when cold air over a warm water (lakes)• Heat and moisture are transferred from the warm water to

the cooler, drier air and the transfer occurs in a shallow layer near the lakes surface

• Hence, you have an unstable situation with warm, saturated air at the lake's surface below cooler air....

• the air rises forming steam fog

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Even in summer, warm air rising above thermal pools in Yellowstone National Park condenses into a type of steam fog.

Fog That Forms by Mixing

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Distribution of Foggy Weather around the U.S.• Where is it foggy?

– Pacific Coast – Appalachian highland region – New England

• Foggiest spot in the U.S.: Cape Disappointment, WA – it's foggy for 2556 hours per year, or about 107 days.

• Other foggy locations: – Mistake Island - off coast of

Maine - 1580 hours per year – Mt. Washington!! - experience

fog on about 300 days each year!

• Fog can be a significant forecast problem:– aviation – ground transportation – maritime activities

Introduction to cloud types• Introduced in 1803 by Luke Howard, an English naturalist;

modified by Abercromby and Hildebrandsson in 1887.• Clouds are comprised of liquid droplets of various sizes and/or

ice crystals • They are characterized according to their height location in the

atmosphere and their vertical development:

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Cloud base height changes with latitude !

High Clouds - Cirrus (Ci)• High clouds are comprised

largely of ice• Cloud-base heights for high

clouds:– Tropical Region (6-18 km) – Middle Latitudes (5-13 km) – Polar Regions (3-8 km)

• Cirrus Clouds - high, thin wispy clouds up at jet stream level in the upper troposphere

• They are almost always comprised of ice

• associated with fair weather

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Cirrostratus Clouds• High, thin, sheet-like clouds • Produce halos around the sun/moon • Rain or snow is often 12/24 hours away

• High, thin clouds appear as small, rounded, white puffs.

• May occur individually or in long rows.• When in rows, is has a rippling appearance.

Cirrocumulus clouds.

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Middle Clouds - Altocumulus (Ac)• Middle clouds are composed of water and/or ice • Cloud-base heights for middle clouds:

– Tropical Region 2-8 km – Middle Latitudes 2-7 km – Polar Regions 2-4 km

• Altocumulus Clouds - shallow, puffy or wave-like in appearance

• appear the size of your thumbnail when holding your arm up to the sky

Middle Clouds - Altostratus (As)• Altostratus Clouds - grayish/blue-gray • thin layer covering entire sky uniformly • found ahead of storms • can seen the sun through altostratus, but NO

halo will be observed

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Low Clouds - stratus (St)• Cloud-base heights for middle clouds:

– Tropical Region 0-2 km – Middle Latitudes 0-2 km – Polar Regions 0-2 km

• Stratus Clouds - Uniform grayish cloud covering the entire sky

• Common in the Northeast Kingdom

Low Clouds - Nimbostratus (Ns)• Nimbostratus Clouds - darker gray, "wet" looking

low clouds • they produce light/moderate precipitation over a

large region

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Low Clouds - Stratocumulus (Sc)• Stratocumulus Clouds - low, lumpy, puffy clouds

in patches or rounded masses • Visually appear larger than altocumulus • Appear the size of your fist when holding your arm

up to the sky

Vertically Developed Clouds - Cumulus (Cu)• Cumulus Clouds - Look like cotton

balls/cauliflower in the sky • sub categories of cumulus:

– cumulus humilis - slightly developed Cu – cumulus congestus - moderately developed

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Vertically Developed Clouds - Cumulonimbus (Cb)• Cumulonimbus Clouds - thunderstorms

– develop from growing Cu – can extend up to the troposphere – can contain both water and ice

• contains precipitation (rain, snow, hail, etc)

• produce lightning and severe weather

• form a distinctive "anvil" cloud at the top of the storm

Anvil

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Summary of Cloud Types

Other unusual clouds - Lenticular Clouds

• Lenticular Clouds - form as air flows over mountains

• look like pancakes, UFOs even

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Other unusual clouds - Cap CloudCap Cloud - forms as air is forced up and

over a mountain

Other unusual clouds - Pileus• Pileus - forms as a growing thunderstorm

deflects moist air up and over the top of the building cumulus congestus or cumulonimbus

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Other unusual clouds - Mammatus• Mammatus - look like cow's udder • visually very impressive • usually form underneath Cb• visual manifestation of sinking air, not rising air!

Other unusual clouds - Contrails• Contrails: are formed from the exhaust of high-

flying aircraft • are similar in makeup to cirrus clouds • have an important impact on the radiation balance

for the earth.

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Nacreous clouds. • They form in the stratosphere and are

most easily seen at high latitudes.

Noctilucent cloudsThey are usually observed at high latitudes, at altitudes between

75 and 90 km above the earth’s surface.

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Other unusual clouds - Scud

• Scud - ragged low clouds beneath actual cloud base

• Often form due to turbulent mixing of air:– warm air from the updraft– cool air from the downdraft

Ground-Based Description of Sky Conditions

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Cloud base measurements with laser ceilometer

Satellite Observations of Clouds• First Meteorological Satellites were launched in late

1950's • Today, we use two types of satellites to monitor

weather: – Geostationary– Polar orbiting

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Geostationary Satellites - GOES• Geostationary satellites are positioned over the equator• They orbit the earth at the same rate as the earths rotation• This allows for a continuous view of the same location• The U.S. has two geostationary satellites:

– GOES-E -->>• located at 0°N, 75°W

– GOES-W -->>• located at 0°N, 135°W

http://goes.gsfc.nasa.gov/

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Q: What are the advantages and disadvantages of the GOES satellites?

Polar Orbiting Satellites• Orbit in a nearly N-S manner around the globe • Nearly pass over the poles • 1 orbit takes about 120 minutes • Q: What are the advantages and disadvantages

of the polar orbiting satellites?

Time 1 Time 2

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Types of Satellite Data• Satellites collect two primary types of

meteorological imagery:– Visible

• basically a picture of the clouds - much like taking a picture with your camera

– Infrared - tells you the temperature of what the satellite is sensing, whether its the ground or the tops of clouds

• very useful for determining the height of the cloud (low, middle, high)

Infrared image of the eastern Pacific

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A visible image of the eastern Pacific

Ocean surface temperature measurements with satellite

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Infrared water vapor image

Get Hurricane cloud and precipitation structure from Satellite